Neutron dosimeter including a step wedge formed of an alpha-attenuating material

ABSTRACT

A neutron dosimeter includes a material which emits alpha particles when bombarded with neutrons, a plastic foil in position to intercept these alpha particles and a step wedge of a material, capable of attenuating the alpha particles, disposed between the alpha-emitting material and the plastic foil. The step wedge varies from zero thickness to such a thickness as will attenuate the most energetic alpha particles formed in the dosimeter to particles having an energy such that the particles form tracks in the plastic foil. Following exposure to neutrons the plastic foil is etched to expose the alpha tracks in the plastic. The location of the tracks in the plastic indicates the energy of the neutrons and the total number of tracks in the plastic indicates the intensity of the neutron flux.

United States Patent [72] Inventors Jacob Kastner 2,938,121 5/1960Fitzgerald et al. 250/83 PH Downers Grove; 3,335,278 8/1967 Price et al250/83.1 Bi G-oltmamworth. both 3,372,275 3/1968 Kocher..... 250/83.l[21] Appl. No. 883,737 3,418,472 12/1968 Evans 250/83.1 [22] Filed 083-10, 1996791 FOREIGN PATENTS [45] Patented Sept. 14, 1 [73] Assignee TheUnmd sums America as 76,602 5/1919 Germany 250/83 PH represented by theUnited States Atomic Primary Examiner.lames W. Lawrence EnergyCommission Assistant Examiner-D. C. Nelms Attorney-Roland A. Anderson[54] NEUTRON DOSIMETER INCLUDING A STEP ED F N ALPHA-ATTENUATING n figggfig 0 A ABSTRACT: A neutron dosimeter includes a material which emitsalpha particles when bombarded with neutrons, a plastic 4 Cla|ms, 3Drawing Figs.

foil in position to intercept these alpha particles and a step U.S. R,wedge of a material capable of attenuating the alpha par. 250/83 CDcles, disposed between the alpha-emitting material and the [51 Int. ClG0lt 3/02 plastic f iL The step wedge varies f Zero thickness to Such{50] Fleld of Search 250/83. 1, a thickness as will attenuate m mostenergetic alpha particles PH, 62, 63, 65, 83 CD formed in the dosimeterto particles having an energy such that the particles form tracks in theplastic foil. Following ex- [56] Reterences cued posure to neutrons theplastic foil is etched to expose the UNITED STATES PATENTS alpha tracksin the plastic. The location of the tracks in the 2,624,846 I/1953Tochlin et al 250/83 PH plastic indicates the energy of the neutrons andthe total 2,715,684 8/1955 Schwarz 250/108 number of tracks in theplastic indicates the intensity of the 2,933,605 4/1960 Ross 250/83.1neutron flux.

I 0 I l l l l L I ll "l l 7 l I..- 11 Z l I -l l --4 l I l I I s. L? L sPATENIED SEPI 412m Fi -E Jacob Kdfiifief Bill? 6 Oltman NEUTRONDOSIMETER INCLUDING A STEP WEDGE FORMED OF AN ALPI-IA-ATTENUATINGMATERIAL CONTRACTUAL ORIGIN OF THE INVENTION The invention describedherein was made in the course of, or under, a contract with the UNITEDSTATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates to a neutrondosimeter and to a method for its useiln more detail, the inventionrelates to a stable neutron dosimeter responsive to neutrons of allenergies while being insensitive to other forms of radiation. In stillmore details, the invention relates to a personnel neutron spectrummonitor capable of indicating the energy of neutrons incident thereon aswell as the total neutron flux.

Dosimeters for the detection and measurement of radiation doses areroutinely used in any location where personnel will be or may be exposedto radiation. Dosimeters are used for the detection of ionizingradiation and for the detection of neutrons. Neutron dosimeterspresently available do not monitor the entire neutron spectrum toindicate the energy level of the neutrons striking the meter, althoughthey may discriminate broadly between thermal neutrons and fastneutrons. While the energy level of neutrons can be determined bycounting and measuring proton tracks in film exposed to neutrons sincethe length of the track is a function of neutron energy, the techniquedoes not lend itself to personnel radiation exposure monitoring due tolatent image fading. In addition, the film used is light-sensitive.Since the relative biological effect of neutron radiation is a functionof the neutron energy, it is certainly highly desirable that the energylevel of the neutrons striking an individual be known as well as thetotal number of neutrons to help in determining whether treatment forradiation exposure is necessary and, if so, what treatment should beused.

SUMMARY OF THE INVENTION According to the present invention, a stepwedge of gold or other alphaattenuating material is interposed between afoil formed of cellulose acetate butyrate, cellulose nitrate or otherplastic material and a crystal of lithium-6 fluoride, a button ofboron-l0, borax formed from boron-10, or other material which emitsalpha particles when struck by neutrons. The meter is checked forneutron exposure by etching the plastic foil with sodium hydroxide orother known etchants to expose the alpha tracks in the plastic resultingfrom neutron events in the lithium-6 or boron-l0. The total number oftracks in the foil is a measure of the intensity of the neutron flux andthe number of tracks beneath each step of the step wedge indicates therelative proportion of neutrons of different energy levels.

DESCRIPTION OF THE DRAWING FIG. 1 is a top plan view of a neutronspectrum dosimeter according to the present invention.

FIG. 2 is a vertical section thereof which is not drawn to scale.

FIG. 3 is a perspective view of the step wedge, not drawn to scale.

DESCRIPTION OF THE PREFERRED EMBODIMENT The neutron dosimeter of thepresent invention includes a housing having a clip 11 fastened to theback thereof and an opening 12 in the front thereof. Within housing 10and facing opening 12 is a single crystal l3-or mosaic of singlecrystals-of lithium-6 fluoride about 2 mm. thick. At the back of housing10 is a foil 14 of cellulose acetate butyrate (CAB) about 10 or micronsthick and disposed between crystal l3 and foil 14 is a step wedge 15 ofgold of thickness ranging from A: micron to 5 microns in steps of 9amicron. Included in crystal 13 is a portion 16 which is not covered withgold. Such an arrangement makes it possible to discriminate amongneutrons having energy levels between thermal and I0 mev.

In use, the device is worn on the clothing of anyone who will or may besubjected to neutron radiation. Following subjection to such anenvironment-or at specified intervals-the dosimeters are collected andthe housing removed therefrom. The CAB foil is removed and etched with,for example, 6.25 N sodium hydroxide at 62 for 2% hours. Etching exposesthe alpha tracks therein and the total number of tracks in the foilindicates the intensity of the neutron flux. The location of the tracksin the foil indicates the energy of the neutrons, as will next beexplained.

If a neutron strikes a lithium-6 atom, an alpha particle is given offand the energy of the alpha particle will be dependent upon the energyof the neutron. For example, a thermal neutron will cause the emissionof an alpha particle having an energy of about 2.0 mev. Neutrons ofhigher energies will induce the formation of alpha particles of higherenergies, since the energy of the neutron is added to that resultingfrom the radiation process. The energy of the alpha particles will bereduced slightly in the lithium-6 unless a very thin crystal is used andmost of the particles induced by thermal neutrons entering the CABsurface will be below about 1.6 mev. We have found that the upperthreshold for appearance of alpha tracks in CAB is about 1.6 mev. Thus,at the end of the plastic foil which was not covered by gold, alphatracks will appear, provided thermal neutrons were incident upon thedosimeter. Alpha particles caused by the incidence of higher energyneutrons upon the dosimeter will not be detected in this portion of theplastic foil since their energy is above the threshold energy forformation of alpha tracks. At the other end of the plastic foil,low-energy alpha particles resulting from the impact of low-energyneutrons upon the detector will not be detected since they will not havesufficient energy to penetrate the relatively great thickness of goldbetween the lithium-6 fluoride crystal and the plastic foil.Higher-energy alpha particles resulting from the impact of higher-energyneutrons upon the dosimeter will be detected since the energy of thesealpha particles will be reduced below the threshold energy for formationof alpha tracks. Similarly, a different energy of neutrons will bedetected below each step of the step wedge of gold. Since each neutronstriking a lithium-6 atom causes the formation of an alpha particle, thedosimeter is very sensitive and can be used in any location where ameasurable neutron flux is or may be present.

A variation on the above-described procedure is to etch the CAB foil insitu; that is, without separating it from the lithium-6 crystal and thegold step wedge. The only tracks that become visible are those thatcompletely penetrate the plastic foil. Useful results can be attained inthis manner if the foil is sufficiently thin that a high proportion ofthe tracks will penetrate the foil.

Although the step wedge may be formed of gold foils of differentlengths, the preferred method of forming step wedge 15 is by vapordeposition of gold on the surface of lithium-6 crystal 13. This is doneby masking a small portion 16 at one end of the crystal andvapor-depositing 5: micron of gold on the crystal. A wider mask is thenused and another 1% micron of gold vapor-deposited on the crystal. Thisis continued till the desired number of steps are formed in the stepwedge. Desirably, the step wedge should contain about 10 steps, eachabout 1 cm. wide and each differing in thickness from the next step by'15 micron to make it possible to discriminate among neutron energylevels between thermal and 10 mev.

Plastic foil 14 may, in addition to cellulose acetate butyrate, beformed of other plastic materials such as cellulose nitrate (CN).Practically, crystal 13 will be formed of an array or mosaic of singlecrystals fitted closely together. If crystal 13 is too thick, alphaparticles will be lost before they get out of the crystal. It must,however, be thick enough to reduce the energy level of alpha particlesformed by thermal neutrons below the threshold for formation of tracks.As has been stated, 2.0 mm. is satisfactory.

Other step wedge material may also be used, although gold is verydesirable because of its chemical inertness and because its thicknesscan be controlled easily. Other candidate materials are aluminum orMylar-a strong transparent plastic that can be obtained in very thinsheets (the order ofa few microns or more).

In addition to lithium-6, other materials which emit alpha particleswhen struck by neutrons may be used. For example, crystal 13 may bereplaced by a button of boron-l or a button of borax, Na B O,-l0H O. Theboron button must be polished, since otherwise crystals of powder wouldprick tracks into the plastic. When boron-l0 is the target material, thelithium-7 recoil nuclei will also produce tracks.

We have carried out measurements of the upper energy threshold for alphaparticles using a thin americium-24l source. One method involvingkeeping the source-recorder distance fixed and varying the air pressurewas used with Diacel CN and another method involving varying thesourcerecorder distance in air at constant pressure was used with CAB.

For the Diacel CN the source-recorder distance was 5.65 cm. The angle ofincidence of the alpha particles on the plastic was -45.Thesource-recorder combination was placed inside a chamber so that the airpressure could be regulated as desired. The main group of alphaparticles from americium-241 have an energy of 5.48 mev. The source usedhad an activity of 2X10 dis/min. Strips of CN were exposed for 90minutes at 26 C. and pressure of 30.5, 33.0, 35.6, 38.1, 40.6, 43.2, and45.7 cm. of Hg. These correspond to incident alpha particle energies atthe CN surface of 3.0, 2.8, 2.5, 2.2, 1.9, 1.5, and 1.1 mev.,respectively. (Based on the range-energy curve given by M. S. Livingstonand H. A. Bethe in the Reviews of Modern Physics 9, 245 (1937).). Goodtracks were obtained up to an alpha energy of 1.9 mev., and no trackswere visible at all above 2.5 mev.

For the CAB similar measurements indicated that the diameter of the etchpits decreased with increasing particle energy and were no longervisible for alpha particles above 1.6 mev.

Source-recorder combinations were irradiated to slow neutrons in thestandard sigma pile at the Argonne National Laboratory. Calibrationswere made by sandwiching a disk of CAB between a gold foil and a LiF orB source. These sources had a smooth flat surface so that good contactwas possible. The Lil crystals were square, 2 cm. on a side and 2 mm.thick. Both sources were black to slow neutrons, and were infinitelythick relative to the range of all charged particles produced by slowneutron capture in Li or B. The recordersource combinations were placedin aluminum holders under pressure supplied by a thin layer of spongerubber to insure good surface contact. In some exposures the packageswere placed in cadmium boxes so that a correction for epithermalneutrons could be made.

The unperturbed thermal neutron flux in the standard pile at theposition where the packages were placed was assumed to be (4,100L200)neutrons/cm. sec. The ratio of the Au activity for a given exposure timewith and without the Lil was 0.42:0.02. The corresponding ratio for theB was 0.50:0.01. The lack of flux depression in the case of the 3" isdue to its small mass. Thus, the thermal neutron flux at the Lil surfacewas 1,720i) neutrons/cm. sec., and the corresponding flux at the B was(2,050i100) neutrons/cm. sec.

The track recording rate for the LiF was (9.65:0.23) tracks/cm. sec. inthe neutron flux of 1,720 neutrons/em. sec. The detection efficiency isthus (5.6103 l0 tracks per thermal neutron. The corresponding numbersfor the B are (26.7il.1) tracks/cm. sec. in the thermal neutron flux of2,050, or l.30- -0.08) l0 tracks/thermal neutron.

The above-calculated efficiencies are for thermal neutrons incident overa 211- solid angle. For thermal neutrons incident isotropically, theabove values should be divided by two, since all neutrons incident onthe back side of the UP and B sources are absorbed without Eroducingtracks.

lt Wlll be understood that t e invention 15 not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A neutron dosimeter including a housing containing a material capableof emitting alpha particles when bombarded with neutrons, a plastic foilcapable of registering etchable tracks from said alpha particles andpositioned to receive them, and a step wedge formed of analpha-attenuating material positioned between the alpha-emittingmaterial and the plastic foil so as to leave a portion of the spacebetween the alpha-emitting material and the plastic foil free ofalphaattenuating material.

2. A neutron dosimeter according to claim 1 wherein said step wedge isformed of gold, the material capable of emitting alpha particles whenbombarded with neutrons is lithium-6 fluoride and the plastic foil isselected from the group consist ing of cellulose acetate butyrate andcellulose nitrate.

3. A neutron dosimeter according to claim 2 wherein said step wedgecontains 10 steps, each differing in thickness from the next by kmicron.

4. A neutron dosimeter according to claim 1 wherein said step wedge isof gold, the material capable of emitting alpha particles when bombardedwith neutrons is boron-l0 and the plastic foil is selected from thegroup consisting of cellulose acetate butyrate and cellulose nitrate.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 60M931 Dated September 1. L, 1971 Invento Jacob Kastner and Billie G. OltmaIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

In column 3, line #7, after "thick." insert the sentence:

--Ihe B O was in the form of circular disks, 8 mm in diameter and 2 mmthiCk.-.

In column line 15, change "10 to --lO line 18, change 10 to --lO Signedand sealed this 2nd day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. ROBERT GO'ITSCHALK Attesting Officer Commissionerof Patents

1. A neutron dosimeter including a housing containing a material capableof emitting alpha particles when bombarded with neutrons, a plastic foilcapable of registering etchable tracks from said alpha particles andpositioned to receive them, and a step wedge formed of analpha-attenuating material positioned between the alpha-emittingmaterial and the plastic foil so as to leave a portion of the spacebetween the alpha-emitting material and the plastic foil free ofalpha-attenuating material.
 2. A neutron dosimeter according to claim 1wherein said step wedge is formed of gold, the material capable ofemitting alpha particles when bombarded with neutrons is lithium-6fluoride and the plastic foil is selected from the group consisting ofcellulose acetate butyrate and cellulose nitrate.
 3. A neutron dosimeteraccording to claim 2 wherein said step wedge contains 10 steps, eachdiffering in thickness from the next by 1/2 micron.
 4. A neutrondosimeter according to claim 1 wherein said step wedge is of gold, thematerial capable of emitting alpha particles when bombarded withneutrons is boron-10 and the plastic foil is selected from the groupconsisting of celLulose acetate butyrate and cellulose nitrate.